For the most part, all of the rules for die opening and air formed radius still apply with plate. But, when it comes to high strength steels, those traditional rules for determining ]minimum bend radius, minimum punch radius, die opening, bending force, and tooling requirements may not always work.
What are the special considerations that you need to take into account?
You’ll need to ask the following questions:
- What is the tensile strength?
- What is the yield strength?
- What is the minimum bend radius?
- What is the recommended punch radius?
- What die opening is correct for the material thickness?
- What effect does grain direction have on forming this material?
When a shop is bending large metal plates they will need some kind of material handling equipment, usually in the form of a crane. There may be some sort of front lifting device ranging from front supports to sophisticated lifting mechanisms.
Typically, a small bend will be made in the plate, the angle can be measured and the controller adjusted, the part is re-hit, checked again… until the bend angle is reached.
When a plate is 20 feet long and weighing thousand pounds, it will take more than one machine operator along with a crane to produce the part. The forklift or crane can help position the part into the location and against the backstops.
Unlike sheet metal forming where only a light touch is necessary, large press brakes are equipped with heavy-duty backgauges. Caution is required when adjusting the plate; because of the sizes and weights of the plate, the backgauges can be knocked out of position by aggressive material handling.
There are programmable “pushing” assistance features on a few material handling devices which can help prevent the rough and aggressive handling.
This is especially important when forming thick and/or high tonnage because thicker and high-tonnage bending is almost never a single-hit operation. The first hit needs to be exactly where it needs to be while ensuring you are able to re-hit the part in the same location. It is common practice to “creep” up on the bend angle with multiple hits. Caution needs to be used; if the part is bent too far and becomes over-bent, it becomes scrap, too large to flip and flatten.
Springback being the tendency of a metal to attempt a return to its original shape after forming. Springback ensures that multiple bends are required for most plate forming jobs. Again, bend too far and the part may be lost.
The high levels of springback associated with heavy or high strength material can make the job a real challenge. In sheet metal forming where the inside radius is one-to-one with the material thickness, springback is only .5 to 1°.
Typically, sheets for a particular job likely came from the same coil or batch and therefore tend to have similar characteristics; the same cannot be said for the plate. Steel mills tend to produce thick plate and/or high-strength material in small batches, so plate characteristics vary from one plate to another.
How much springback is common in a plate or high-strength steels? With the mild cold rolled steel, 60,000 tensile plate it is likely the material will springback 3 to 5°; high-strength steels can be quite a bit higher, 10 or 20° of springback is not unheard of. Springback is the main reason the bend angles are crept up on, relieving the springback a little bit at a time.
Some High strength steel (HHS) requires that bends be created in a single stroke rather than creeping up on the bend. These materials must not be allowed to springback during the stroke the way it does during re-hitting.
To verify whether this is the case for the material being formed, check with the service center or check the internet for that information. With that knowledge, adjustments to the press brake’s automatic springback compensation device can be made easily.
When reviewing the material specifications note the yield strength of the material. The yield strength of a given material has a tremendous influence on the amount of springback. The higher the yield strength is, the greater the amount of springback that will be encountered.
Using tribal knowledge operators were taught to use a die opening that is equivalent to ten, twelve even 20 times the metal thickness as a starting point. This is a variation of the “eight times” rule and is not accurate; geometrically perfect die openings can be easily found using the formula given in the die chapter. But that selection method really relates to sheet metal instead of the plate.
When bending plate or high-strength steel, you will need something much larger. If the true general rule-of-thumb for sheet metal is used for plate, it is likely that cracking of the material will occur on the outside surface of the workpiece.
When forming plate or high-strength steel, standard reference charts are a good resource. They can help to determine the proper die opening and resulting inside radius. For plate or high-strength steel, there is very little chance that bottoming or coining will be used because of the high tonnage requirements. Consequently, you will be air forming producing the inside radius as a percentage of the material thickness, the 20% rule.
Rather than a multiple of eight times the material thickness, the die opening for a plate or high-strength steel will use a multiplier of 12 or 14 times the material thickness gradually increasing with thickness. And, the final inside radius will be a percentage of the die opening as expressed by the 20% rule.
Bending objects this large really doesn’t lend itself to quick actions, and tooling changes also fall into this category. Manually wrestling a 20-ft. long punch is both awkward, time-consuming and dangerous.
One option to the standard die design is the adjustable die; this offers unlimited die openings without actually having to remove and replace the tooling between parts, figure 4.
Of these adjustable dies, there are two very similar designs; both are infinitely adjustable through the use of shims, and some are adjusted using wedges or worm drives. They each vary in the way the material interacts with the top two corners (the die radius) of the tool. One uses hardened steel inserts, figure 4. The material is dragged over these fixed edges, in turn, causing galling on the outside surface of the workpiece.
The second is the same basic tool, but instead of a hardened steel insert, these have hardened steel rollers. See the video; courtesy of Wila Tool.
Courtesy of Wila Tool
For plate or high-strength steel, it is common practice to use a punch nose at least three times the thickness rather than one-to-one with the material thickness. It is also advisable not to use a sharp punch nose radius to avoid errors created by forcing a ditch into the center of the bend. Applying these rules reduces or alleviates many cracking problems.
Changeable nose radii
Again, due to weight and length, the punches are dangerous to change. Because plate steel bending rarely ever has a down flange involved, both the punch and die are usually full length. To avoid having to remove them, the punch nose of some tools can be slid on and off a dovetail, figure 5.
Grain direction can have a profound effect on any forming operation, but even more so for the plate; forming with or across the grain, figure 6.
The fact is that forming first with and then against the grain produces different bends, even when cut from the same blank. The inside radius will be slightly different and the bend angle will change. This effect is true regardless of gauge or thickness.
There is another factor to consider; if the bend is with the grain it will have a tendency crack on the outside radius of the bend. A cross grain bend is also known as a Transverse bend, and a bend that is the grain with the grain is considered a Longitudinal bend, figure 7.
For smaller symmetrical parts made from plate steel, always orientate the workpiece so the bend is across to the grain.
If the part is not symmetrical, and if possible cut the part so the grain direction is diagonal to the grain.
The required tonnage and the amount of cracking can be reduced by heating the area prior to forming. Raising the temperature to just 200 or 300° can reduce cracking at the bend.
Raising the temperature to 900° Fahrenheit will reduce the forming load, but never more than 1050°.
With temperatures ranging from 1600 to 2000° Fahrenheit a major drop in yield strength will occur. Average plate steel will form like butter at this temperature and the higher strength steels will weaken, making forming easier and reduce the machine tonnage required.
But at those higher temperatures (1600 – 2000°) there will be a significant change to the properties of the base metal! This means that the material may need to be treated to bring it back to the original state.
It is also wise at any temperature not to let the material remain hot for too long because the material can oxidize.
Figure 8 is a chart that offers you a rough idea of where your temperature is based on color; it is not perfect, but it serves the purpose.
Be sure that if heating is required, the temperature of the workpiece is uniform across the entire bend line. Heat the part with a rosebud tip with Oxy-Acetylene mix. Do not use a cutting head, removing material is not the goal.
Heating the part also works on many alloys other than steel.
Depending on the thickness there are several ways to cut steel plate: a torch, plasma, laser or WaterJet. If the thickness is thin enough, shearing may also be an option.
When a shear is employed, the edge is subject to fracture and breakaway. So the edge may be tapered back several degrees. That taper can make gauging and measuring difficult.
Plasma or torch cutting will not leave a perpendicular edge either. The laser will cut a perpendicular edge, but, like plasma and Oxy-acetylene, it heats the workpiece at the cut, creating what is called the Heat Affected Zone (HAZ), figure 9.
This zone, which can get rather large in thick material, creates an area that has been hardened or heat treated from the cutting. It will form differently in these areas from the rest of the plate; and can also work at the tooling, destroying them over time.
Edge and Surface Condition
Edge condition is an important factor. In cases where the material has been sheared, there may be micro-fracturing at the edge, figure 10. These can open up into cracks on the outside surface of the workpiece. The same is true for the surface of the material if it is rough and scaly, pitted or scared; these, too, can open up into cracks.
It is a good practice to take a grinder and dress theses areas of a very small amount; this will also help the cracking problem. Do not remove large amounts of material the object is to clean up the micro-fractures not to remove a 1/4 – inch of material.
Forming plate or high strength steel not only requires top quality tooling, it requires tooling that can withstand the extreme pressures generated during bending.
Wear on the die shoulder radii increases drag on the material as it flows over the shoulder radii. This significantly increases the amount of force required to bend the material, the nose radius of the punch and the top shoulders of the die must also be hardened.
Always bend the material to the largest radius possible. This will help to reduce the material cracking that may occur on the outside of the bend radius, figure 11.
Grind out all surface scratches and all other surface defects on the outside surface opposite the bend line, as these surface and edge conditions may cause cracks.
Lubrication of the material is often necessary, especially for dies that have the hardened inserts rather than rollers. As the material flows over the die shoulder radii or rolling insert during forming, the lubrication will help to reduce drag during the drawing process.
Lubricants specifically designed for High Strength Steels are available, but even grease and/or wax will work. Regardless of the kind of lubricant, it will help to reduce tooling wear and improve the material surface condition after forming.
Make sure the tooling has an included angle on the punch that will provide adequate clearance to over-bend the material to compensate for springback. Die angle too.
High Strength Steels generally have high yield strength]s, the amount of springback will be greater than springback that occurs when bending softer materials such as mild steel and aluminum.
Crowning System / Anti-Deflection devices
The large amounts of tonnage required to bend High Strength Steel does increase machine deflection in the center of the ram. To compensate for deflection typically requires that a minimum of 60% of the bed at center be used.
As always, safety must be the highest priority. The rated capacity of the Press Brake and the punch and die should always be checked against the bending pressure to be applied during the bending process.
Check the load limits of your press. Not the tonnage it can produce, though that is important, check the ram’s load limit against required tonnage. It is possible to break or bend the ram; this is called “ram upset”.
Always wear your hard hat, gloves and safety glasses; steel-toed shoes are a must.
Never place yourself or any part of your body in a position between the plate and any solid object: wall, machine or a stack of plates. Plate is extremely heavy and it does not take much to cause the load to swing or drift, possibly crushing you or some part of your body.
Never stand, walk or drive under any suspended load, and always make sure that both you and the operator are seen, be that on the brake or around the crane.
Additional recommended reading
October 2014: Fundamentals of heavy bending
After reviewing this material you should now be able to:
- Define the difference between sheet metal and plate.
- Discuss the effects of springback on plate.
- Use adjustable channel dies.
- Use tooling with “dove tail” mounted punch radii.
- The effects of grain direction.
- Explain why sharp or minimum radius bends are more likely to suffer cracking.
- Explain why transverse bends are preferred to longitudinal bends.
- Correctly heat the material to reduce the tonnage requirements.
- Explain HAZ, the heat affected zone and what it does for the forming process.
- Discuss crane safety.
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